In the current state of the art in threaded connections for oil field tubular goods, it is important to have relatively easy interconnectability, good mechanical properties and also resistance to leakage under high internal pressures and high tensile strains. Preferably this is to be done without incurring substantial cost penalties relative to tubular goods standardized in accordance with specifications set by the American Petroleum Institute (API). Drilled oil and gas wells are becoming of increasingly longer length, are arranged in complex geometries, and are often directionally controlled to curve to a production zone. In addition they are likely to be required to operate in corrosive environments at increasingly higher pressures and temperatures. It sometimes is unavoidable to have to use “premium” tubing with special threads or seals, or both, to meet the strength and leak resistance specifications that are needed.
In a drilled bore hole for petroleum production an outer string of relatively larger diameter tubular elements, called the casing, is joined together by using interspersed couplers. The casing is left in place after the well is completed, and is externally sealed by surrounding cement. It must have physical integrity under the high loads, forces, pressure and corrosive conditions that may be encountered. Joints in the casing string are particularly subject to failure. The size and weight of the casing are considerable, but the installation, being essentially static, and usually encased in concrete, is relatively undisturbed once in place. However, leakage of high pressure gas into the annulus between the inner tubing and the casing can give rise to external environmental damage if it penetrates outside the casing. Self-sealing interconnections in the casing can limit or eliminate this problem, which can create major environmental and economic consequences.
A string of production tubing of smaller diameter is installed within the length of the casing, and may repeatedly be withdrawn and disassembled for service or replacement. Petroleum products can be lifted upwardly by down hole pressure alone, or more typically by pumping, as with a rotary or reciprocating pump. In either event a sucker rod string extends within the production tubing from the surface down to a pump in the production zone. For flow control, down hole packers, sensors and other elements may be located in and adjacent the production zone.
Heat, internal pressures and corrosive chemicals in the deep down hole installations that are now being widely employed militate against mechanically secure leak-free connections. Tensile strains from the weight of long lengths of tubing increase the likelihood of leakage under high internal pressures. Countering these adverse conditions without at the same time introducing major added costs is an objective of this invention.